In vertical venetian blinds, the slats are commonly supported on carriages for movement along a trackway to enable opening and closing of the blind and the slats are also supported on the carriages for rotation above the lengthwise axis of the slat to control the light that passes through the blind. Rotation of the slat carriers in unison is effected by an operating shaft that extends lengthwise of the trackway and which is connected through gearing in each of the carriages to the slat carrier to rotate the slat. In some such vertical venetian blinds, for example as shown in U.S. Pat. Nos. 2,848,045 and 2,993,535, a drive sprocket is connected directly to one end of the operating shaft to rotate the shaft in response to a pull exerted on one or the other of the runs of a flexible chain entrained over the drive sprocket. However, in order to enable more precise control of the angle of the slats, it is desirable in some applications to provide a speed reduction drive for the operating shaft. U.S. Pat. No. 3,280,891 discloses a vertical venetian blind traverse apparatus having an eccentric cam and gear type speed reducer for connecting a drive sprocket to the shaft with a speed reduction. The eccentric cam and gear type speed reducer of this patent, however, produced a non-uniformed rotation of the shaft in response to rotation of the drive sprocket. In addition, it utilized a relatively large number of parts which made it somewhat expensive to fabricate and assemble. Further, in this patent the guide pulleys for the carriage traverse cords were spaced inwardly of the end of the rod to provide clearance for the speed reducer mechanism and therefore required substantial space between the end slat and the window opening in order to provide clearance for manipulating the downwardly extending runs of the carriage reverse cords.
It has also been proposed as shown in U.S. Pat. No. 4,122,884 to provide a worm and gear type speed reducer in which the input shaft extended through the front wall of the trackway at right angles to the operating shaft. This reduces the dead space required at the end of the head rail for the operating mechanism of the venetian blind. However, it is desirable in some installations to provide an operating mechanism which does not extend from the front of the head rail.